The present invention relates to a technique for diagnosing whether a wide-range type air-fuel ratio sensor to be used for an air-fuel ratio feedback control in an internal combustion engine is activated or not.
Heretofore, an air-fuel ratio feedback control method is known where the air-fuel ratio of an engine intake air-fuel mixture is detected indirectly by detecting the oxygen concentration in an engine exhaust through an oxygen sensor, and controlling the fuel supply quantity so that the air-fuel ratio detected through the oxygen sensor approximates a target air-fuel ratio (refer for example to Japanese Unexamined Patent Publication No. 60-240840).
In the above-mentioned conventional air-fuel ratio feedback control, a method is generally performed where an oxygen sensor for detecting the rich/lean of the air-fuel ratio in comparison to the theoretical air-fuel ratio is utilized so as to control a target air-fuel ratio to the theoretical air-fuel ratio. However, in correspondence to the recent increase in demand for the improvement of exhaust emission performance or the improvement of fuel economy, a lean burn engine is being developed having a target air-fuel ratio which is set to a value much higher than the theoretical air-fuel ratio (for example, 20-24). In such an engine, a wide-range type air-fuel ratio sensor capable of detecting a wide range of air-fuel ratio regions is utilized as the oxygen sensor.
Heretofore, generally in such air-fuel ratio feedback control, judgement is made as to whether the oxygen sensor is at an activated state, so that good output characteristics of the oxygen sensor may be gained before starting the air-fuel ratio feedback control based on the output value, thereby performing the control with high accuracy.
In the case of an oxygen sensor for detecting the rich/lean state of the air-fuel ratio to the theoretical air-fuel ratio by an on/off method, the activation status of the oxygen sensor may be judged by the output value (output voltage) being fixed either to the upper limit value on the rich side or to the lower limit value on the lean side.
However, in the case of a wide-range type air-fuel ratio sensor capable of detecting a wide range of air-fuel ratio regions, after the sensor is activated, the detection signal corresponding to the air-fuel ratio neighborhood (for example, the theoretical air-fuel ratio neighborhood) at the time is output, but since the variation range of the output value is small, the determination of the activation status of the sensor is difficult. Therefore, conventionally, it was common to wait for the lapse of a predetermined time after the start of engine operation, in which time the air-fuel sensor is considered to have been sufficiently activated, before shifting to the air-fuel ratio feedback control. However, in the above method, the predetermined time is set to a large value for leaving a considerable latitude, so that the air-fuel ratio sensor is judged to have been sufficiently activated even according to the operating condition where the activation is most delayed. Accordingly, in practice, there is a long period of time until the air-fuel ratio feedback control is started after the activation of the sensor. This is against the aim of using a wide-range air-fuel ratio sensor, which is to improve the exhaust emission performance.
The present invention aims at solving the above problems. The object of the invention is to diagnose the activation status of the wide-range air-fuel ratio sensor with high accuracy.
Further object of the present invention is to start the air-fuel ratio feedback control at an earlier stage, and to improve the exhaust emission performance, by improving the accuracy of the diagnosis.
In order to achieve the above object, a first activation diagnosis method according to the present invention comprises the steps of:
calculating heat transferred to and from a wide-range type air-fuel ratio sensor, an output value of the sensor being varied in response to oxygen concentration in exhaust which varies according to an air-fuel ratio in an intake air-fuel mixture of an internal combustion engine;
estimating the activation time from the starting of operation of the engine until the air-fuel ratio sensor is activated, based on the calculated heat transferred to and from the air-fuel ratio sensor; and
diagnosing that the air-fuel ratio sensor is activated when the estimated activation time has passed after the starting of operation of the engine.
Further, a first activation diagnosis apparatus according to the present invention comprises each device or means for performing each function in the first activation diagnosis method.
According to the first diagnosis method or the first diagnosis apparatus, since it is possible that the heat transferred to and from the air-fuel ratio sensor is calculated, and based on the result of calculation, the heat increase characteristic of the air-fuel ratio sensor is estimated, the time necessary for the air-fuel ratio sensor to be activated may also be estimated. After the estimated activation time has passed from the starting of operation of the engine, the air-fuel ratio sensor is diagnosed as being activated, and an air-fuel ratio feedback control based on the air-fuel ratio sensor can be started.
With this construction, the activation status of the air-fuel ratio sensor may be diagnosed with high accuracy, and the air-fuel ratio feedback control may be started at a considerably early stage, thereby improving the exhaust emission performance.
Further, the estimation of the activation time may be performed based on at least two data selected from an environmental temperature at the starting time of the engine, a heat generation quantity of a heater installed to the air-fuel ratio sensor, and a heat quantity of the exhaust.
Since the environmental temperature at the starting time of the engine relates to a heat radiation quantity from the air-fuel ratio sensor, and the heat generation quantity of the heater and the heat quantity of the exhaust relate to a heat quantity to be supplied to the air-fuel ratio sensor, the time required for the air-fuel ratio sensor to be activated may be accurately estimated based on at least two parameters out of the above three parameters.
Moreover, the environmental temperature at the starting time of the engine may be either an ambient temperature or a cooling water temperature for cooling the engine.
Further, the estimation of the activation time may be calculated by the following equation:
activation time T=Toxe2x88x92TAxe2x88x92TB;
wherein To represents a reference activation time calculated based on the environmental temperature at the starting time of the engine, TA represents an activation shortening time corresponding to the heat generation quantity of the heater, and TB represents an activation shortening time corresponding to the heat quantity of the exhaust.
With this construction, the heat transferred to and from the air-fuel ratio sensor may be calculated as accurately as possible by referring to all of the environmental temperature at the starting time of the engine, the heat generation quantity of the heater and the heat quantity of the exhaust. Thereby, the activation of the air-fuel ratio sensor may be diagnosed with a high degree of accuracy.
In the present invention, the lower the environmental temperature at the starting time of the engine is, the greater the calculated value of the reference activation time To is calculated. The reason for this is because the time needed for activation of the sensor is increased when the environmental temperature is decreased, which leads to increase of the heat quantity to be radiated from the air-fuel ratio sensor.
Further, the activation shortening time TA corresponding to the heat generation quantity of the heater is calculated as a value proportional to the power consumption of the heater. Since the heat generation quantity of the heater is proportional to the consumption of power, the heat generation quantity of the heater may be calculated with high accuracy by multiplying a constant by the consumed power calculated by multiplying the voltage by the current (or the power supply duty in the case of a duty control).
Moreover, the activation shortening time TB corresponding to the heat quantity of the exhaust is calculated by multiplying a basic value set proportionally to an intake air quantity of the engine by a correction coefficient calculated based on the engine rotation speed. That is, the heat quantity of the exhaust being supplied to the air-fuel ratio sensor is basically proportional to a flow quantity of the exhaust when assuming that the air-fuel ratio is constant. Therefore, a basic value equivalent to the flow quantity of the exhaust and proportional to the intake air quantity detected for the control of the engine is calculated, and the basic value is corrected in correspondence to the speed of flow of the exhaust based on the engine rotation speed, so as to gain an accurate calculation result.
With a second activation diagnosis method, there is provided an activation diagnosis method for a wide-range type air-fuel ratio sensor, the air-fuel ratio sensor being equipped with an oxygen concentration detecting unit formed of a solid electrolyte and outputting detection signals corresponding to oxygen concentration inside a hollow chamber to which exhaust of an internal combustion engine is introduced, and an oxygen pump unit for pumping oxygen into or out of the hollow chamber by controlling a current being applied to a solid electrolyte wall separating the hollow chamber and the exhaust side of the engine so as to control the oxygen concentration inside the hollow chamber to predetermined oxygen concentration, the air-fuel ratio sensor detecting the current being applied to the solid electrolyte wall to output an air-fuel ratio signal corresponding to the oxygen concentration in the exhaust; the activation diagnosis method comprising the steps of:
monitoring an output voltage of the oxygen concentration detecting unit after the starting of operation of the engine;
judging whether or not the output voltage of the oxygen concentration detecting unit is fixed to either a value equal to or above a rich-side set voltage or a value equal to or below a lean-side set voltage; and
diagnosing that the air-fuel ratio sensor is activated under the condition that a status is detected where the output voltage of the oxygen concentration detecting unit is fixed to either a value equal to or above a rich-side set voltage or a value equal to or below a lean-side set voltage.
Further, a second activation diagnosis apparatus according to the present invention comprises each device or means for performing each function in the second activation diagnosis method.
According to the second diagnosis method or the second diagnosis apparatus, the oxygen concentration detecting unit functions as an oxygen sensor for judging the rich/lean state of the air-fuel ratio to the theoretical air-fuel ratio by an on/off method.
Therefore, as explained above, while the output voltage of the oxygen concentration detecting unit functioning as the oxygen sensor is monitored, when the output voltage is detected to be in a state fixed either to a value equal to or above a rich-side set voltage or a value equal to or below a lean-side set voltage, the air-fuel ratio sensor may be diagnosed to be substantially activated. Therefore, the air-fuel ratio sensor may be diagnosed as activated after the above judgement, and the air-fuel ratio feedback control based on an air-fuel ratio sensor may be started.
Accordingly, the activation of the air-fuel ratio sensor may be diagnosed with a high degree of accuracy, and the air-fuel ratio feedback control may be started at a very early stage, so the exhaust emission performance of the engine may be improved.
Further, the activation of the air-fuel ratio sensor may be diagnosed when a predetermined time has passed after detecting the status where the output voltage of the oxygen concentration detecting unit is fixed to either a value equal to or above a rich-side set voltage or a value equal to or below a lean-side set voltage.
Actually, the output of the wide-range air-fuel ratio sensor is stabilized when some time has passed for the temperature of the air-fuel ratio sensor as a whole (including the Nernst unit and the like) to stabilize after the oxygen concentration detecting unit functioning as the oxygen sensor is activated. Therefore, the air-fuel ratio sensor is diagnosed to be activated after the above-explained predetermined time has passed, to start the air-fuel ratio feedback control based on the air-fuel ratio sensor, thereby enabling the air-fuel ratio feedback control to be started at an even more stabilized status.
Here, the predetermined time may be set based on the heat transferred to and from the air-fuel ratio sensor. Thereby, the predetermined time needed for the temperature of the air-fuel ratio sensor as a whole to stabilize may be set more accurately, and the activation diagnosis accuracy for the air-fuel ratio sensor may be improved even further.